Mason County, Illinois
A shallow water table limits basements on 42% of Mason County's mapped land, a higher share than 59% of US counties. Shrink-swell clay limits houses without basements on 3%.
None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 9 flood losses here since 2016. 82% of homes were built before 1980, against 50% nationally; the median home dates from 1959.
Data as of October 13, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, October 13, 2026
of homes built before 1980
Soil under Mason County
| Expansive clay (limits slab homes) | 2.9% |
| High water table (limits basements) | 41.7% |
| Septic fields very limited | 88.7% |
| Clay swell (LEP, by area) | 2.4% |
| High-swell land (LEP 6%+) | 0.9% |
| Mapped soil | 360,510 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Plainfield | 11% | 2% |
| Onarga | 7% | n/a |
| Bloomfield | 6% | n/a |
| Sparta | 5% | n/a |
| Udolpho | 4% | n/a |
| Marshan | 4% | n/a |
How we count · survey areas IL125 (2025-08-31)
What the numbers mean here
On 42% of Mason County's mapped land, soil stays saturated within about 6 feet of the surface for part of the year, which is basement depth.
Rain over the 12 months to August 2026: 38.56 in, 0.04 in below the 1991-2020 average year of 38.60 in.
USDA rates 89% of the land very limited for septic absorption fields.
Drought
0% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 24 of the last 52 weeks; the most was 100% of its area, in 24 weeks, most recently on the March 31, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 0% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
9 flood losses since 2016, $83,482 paid. The worst year was 2019, with 4.
| Year | Flood losses |
|---|---|
| 2016 | 4 |
| 2017 | 0 |
| 2018 | 0 |
| 2019 | 4 |
| 2020 | 1 |
| 2021 | 0 |
| 2022 | 0 |
| 2023 | 0 |
| 2024 | 0 |
| 2025 | 0 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 38.60 in |
| 12 months to August 2026 | 38.56 in (-0.04) |
| 2025 | 26.96 in (-11.64) |
| Wettest year since 1895 | 2009: 57.38 in |
| Driest year since 1895 | 1901: 21.82 in |
| Days with 1 inch or more, normal at HAVANA (9 mi away) | 11.1 |
Homes
| Housing units | 6,622 |
| Median year built | 1959 |
| Built before 1980 | 82% |
| Built 2000 or later | 7% |
| Owner-occupied | 82% |
| Median value, owner-occupied | $103,400 |
| Single-family detached | 89% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Menard County, IL | 16 mi | 27% | 51% |
| Fulton County, IL | 22 mi | 24% | 38% |
| Cass County, IL | 26 mi | 13% | 45% |
| Tazewell County, IL | 28 mi | 33% | 53% |
| Logan County, IL | 30 mi | 45% | 71% |
| Sangamon County, IL | 36 mi | 58% | 72% |
| Schuyler County, IL | 37 mi | 31% | 46% |
| Peoria County, IL | 39 mi | 29% | 37% |
Sources: USDA SSURGO (August 31, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status